The invention relates generally to integrated circuit packages, and more specifically to a method of forming a ball-grid array integrated package at a wafer level.
The footprint of an integrated circuit package on a circuit board is the area of the board occupied by the package. It is generally desired to minimize the footprint and to place packages close together. In recent years, the ball-grid array (BGA) package has emerged as one of the more popular package types because it provides high density, minimum footprint, and shorter electrical paths, which means that it has better performance than previous types of semiconductor packages.
A typical BGA package is shown in FIG. 9. In the BGA package 110, an integrated circuit chip 122 is mounted by means of an adhesive on an upper surface of a base 112 made of a substrate material. Metal bonding wires or wirebond leads 120 electrically connect a plurality of metal chip pads 126 formed on the upper surface of the chip 122 with wire bonding pads 128 formed on the upper surface of the base 112. The base 112 includes plated through-hole vias 118 and metal traces 114 to connect the circuitry from the upper surface to the lower surface of the base 112. A plurality of solder balls 116 are placed on the bottom surface of the base 112 and are electrically connected to the metal traces 114 of the base. The solder balls 116 can be arranged in a uniform full matrix array over the entire bottom surface, in a staggered full array, or around the perimeter of the bottom surface in multiple rows. The solder balls are then used to secure the chip package onto a printed circuit board in the end-use product.
While the BGA packages of the prior art provide a great improvement over earlier types of packages in terms of high density and high I/O capability, it is always desired to make the IC package even smaller to further decrease the amount of space needed on a printed circuit board to accommodate the package. Because the wirebond leads are of a predetermined length and require a minimum spacing between adjacent bonding sites to provide sufficient room for the bonding tool, the substrate base must be larger than the chip and it is not possible to fabricate a more compact package. Ideally, it is desired to make a package in which the substrate base does not have to be any larger than the size of the chip.
In the prior art, as described above, it is common to fabricate a package for each individual die. Others have realized that it would be advantageous to be able to form the IC package at the wafer level, that is, after the individual chips have been formed on the wafer but before the wafer has been diced into individual chips. This allows for easier mass production of chip packages and for several chip packages, arranged in a matrix format on the wafer, to be manufactured and tested all at one time. This can reduce time and cost in the process of packaging and testing IC chips.
Some examples of packaging methods in the prior art that are conducted at the wafer level include: U.S. Pat. No. 5,604,160 to Warfield, which discloses using a cap wafer to package semiconductor devices on a device wafer; U.S. Pat. No. 5,798,557 to Salatino et al., which describes a wafer level hermetically packaged integrated circuit having a protective cover wafer bonded to a semiconductor device substrate wafer; and U.S. Pat. No. 5,851,845 to Wood et al., which discloses a method of forming a semiconductor package by providing a wafer containing a plurality of dice, thinning a backside of the wafer by polishing or etching, attaching the thinned wafer to a substrate, and then dicing the wafer.
It is the object of the present invention to provide a method of forming a ball-grid array IC package that has a minimum size such that the IC package takes up no more space than the area of the IC chip.
It is a further object of the invention to provide a method of forming such an IC package at the wafer level in order to take advantage of the greater efficiency in mass production and the ability to conduct parallel testing of the IC packages.
The above objects have been achieved in a method of forming an integrated circuit package on the wafer level using a flip chip design with a single wafer. The integrated circuit package is formed by first providing a product silicon wafer having a plurality of microelectric circuits fabricated thereon and having a plurality of standard aluminum bonding pads exposed. The aluminum bonding pads are re-metallized to be solderable. Then, a layer of adhesive is deposited onto the wafer surface, the bonding pads remaining exposed. A pre-fabricated interposer substrate, having metallized openings, is aligned to the wafer and then the assembly is cured. Solder, or conductive adhesive, is then deposited through the openings in the substrate and the assembly is reflowed, or cured, to form the electrical connection between the circuitry on the substrate and the bonding pads on the silicon wafer. Solder balls are then placed on the metal pads on the substrate and are then reflowed forming a BGA structure. The wafer is then diced and the individual BGA packages are formed. The BGA package is flipped for mounting on a circuit board.
The integrated circuit package of the present invention is smaller than BGA packages of the prior art in that the additional space usually required because of the use of wirebonding leads is not necessary. The whole wafer can be packaged all at one time which is more efficient than packaging each die individually and allows for parallel testing of the packaged dice while still in wafer form.